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[MX] Configuring the interface-mode trunk does not change the interface MTU to 1518

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Article ID: KB27031 KB Last Updated: 06 Mar 2013Version: 1.0
Summary:
This article describes the issue of the MTU on the interface remaining at 1514, instead of changing to 1518 bytes, when the interface is configured to operate in the trunk mode.
Symptoms:
  • This issue exists for transit traffic, when a ping with a packet size of 1469 or greater is attempted.

  • So, the total packet size is 1469 bytes + 20 byte IP header + 8 byte ICMP header + 4 VLAN tag + 14 bytes of Ethernet header = 1515, which is greater than the interface MTU of 1514 on the link.

  • Changing the MTU size to 1518 resolves the issue. However, this should be the default, as this a trunk port.
Cause:
This is day one behavior, as per PR726455.
Solution:
The workaround is to configure VLAN tagging on the interface. The additional 4 bytes accommodates the future addition of a stacked VLAN tag configuration on the same physical interface:
[edit interface ge-1/1/1]
vlan-tagging;
unit 0 {
    family bridge {
        interface-mode trunk;
        vlan-id-list [1-100];
    }
}
You can also manually configure the MTU.

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